The present invention relates to a cryostat and, more particularly, to a cryostat with a refrigerating machine for cooling a superconductive magnet.
A typical prior art cryostat for cooling a superconductive magnet is shown in FIG. 1.
FIG. 1 shows an example of a cryostat of a nuclear magnetic resonance device which is generally referred to as "NMR" and in which a superconductive magnet is used. In FIG. 1, reference numeral 1 denotes a cryostat body; 2, a cylindrical inner wall; 3, a superconductive magnet; 4, a first liquefied gas reservoir (which will be hereinafter referred to as a liquefied helium resorvoir) containing a first liquefied gas (which will be hereinafter referred to as a liquefied helium) for cooling the superconductive magnet 3; 5, a second liquefied gas reservoir (which will be hereinafter referred to as a liquefied nitrogen reservoir) provided around the liquefied helium reservoir 4 in order to prevent a heat leak thereinto and containing a second liquefied gas (hereinafter referred to as a liquefied nitrogen) having a boiling point higher than that of the first liquefied gas; 6, a liquefied helium supply passage; 7, a liquefied nitrogen shield tube; 8, a liquefied helium supply passage cover; 9 a liquefied nitrogen supply passage; and 10, a liquefied nitrogen supply passage cover. A space surrounded by the cryostat body 1, the liquefied helium reservoir 4, the liquefied nitrogen reservoir 5 and the cylindrical inner wall 2 is kept under a vacuum condition in order to reduce the heat leak from the outside. FIG. 2 illustrates a supply manner of liquefied helium and nitrogen in the conventional cryostat, in which reference numeral 11 denotes a liquefied helium container; 12, a liquefied helium supply pipe; 13, a liquefied nitrogen container; and 14, a liquefied nitrogen supply pipe.
The operation of the thus constructed conventional cryostat will be described. First of all, liquefied nitrogen is fully supplied from the liquefied nitrogen container 13 through the liquefied nitrogen supply pipe 14 and the liquefied nitrogen supply passage 9 to the liquefied nitrogen reservoir 5. Subsequently, liquefied helium is fully supplied from the liquefied helium container 11 through the liquefied helium supply pipe 12 and the liquefied helium supply passage 6 to the liquefied helium reservoir 4.
When the liquefied helium is supplied to the liquefied helium reservoir 4, the magnet in the reservoir is held under a superconductive state and will operate as a superconductive magnet 3.
When the superconductive magnet 3 operates, a test piece (not shown) set inside of the cylindrical inner wall 2 is subjected to a magnetic field, enabling to conduct a living body inspection through a nuclear magnetic resonance.
However, according to such a conventional cryostat, there have been raised the following disadvantages. Namely, in an NMR system for a whole human body, which is used for the purpose of a cancer inspection, since the superconductive magnet 3 becomes large in physical size, the liquefied helium reservoir 4 containing it and the liquefied nitrogen reservoir 5 for performing the thermal shield are correspondingly large in size. Therefore, the heat leak into the liquefied nitrogen reservoir 5 and the liquefied helium reservoir 4 would be remarkable. Thus, since evaporation of the liquefied nitrogen and helium is accelerated, the supply amount of the liquefied nitrogen and helium must be increased. The liquefied nitrogen and liquefied helium must be dealt with by a skilled artisan. In addition, since the NMR is installed in a hospital, such an artisan who would not be required in the hospital must be employed only for the purpose of supply of liquefied nitrogen and helium. Moreover, an operation for interchanging liquefied nitrogen containers 13 and liquefied helium containiners 11 is periodically needed inconveniently.